Crossref holds metadata for approximately 150 million scholarly artifacts. These range from peer reviewed journal articles through to scholarly books through to scientific blog posts. In fact, amid such heterogeneity, the only singular factor that unites such items is that they have been assigned a document object identifier (DOI); a unique identification string that can be used to resolve to a resource pertaining to said metadata (often, but not always, a copy of the work identified by the metadata).
This is a companion discussion topic for the original entry at https://0-www-crossref-org.libus.csd.mu.edu/blog/what-do-we-know-about-dois